Gist is facing almost two weeks of strike action from warehouse staff as the result of a pay difference dispute.
Unite said 20 of its members voted unanimously to hold 12 24-hour strikes at the operator's M&S DC in Hemel Hempstead, Hertfordshire.
The pickers and loaders are protesting an £8,000 annual pay gap between them and other members of staff performing the same role.
The Three Cherry Trees site delivers food to M&S stores across London and the South East.
The first 24-hour strike is scheduled for 00.01 on 30 June, followed by 11 more between 7 July and 15 September.
Unite said the 20 affected employees are not part of Gist's national staff agreement, and that making them a part of it to secure equal pay would resolve the dispute.
Unite regional officer Alan Brkljac said: “The kernel of this dispute is that the Gist bosses refuse to recognise our members’ case for equal pay for equal work.
“This dispute could be easily resolved if the 20 plus workers were put on the national agreement that covers 2,000 Gist employees nationwide. The management has continually dragged its feet on this issue and we call on Marks and Spencer to exercise its influence on Gist to assist in the settlement of this dispute."
Gist had not responded to a request for comment at the time of publication.
